The present invention relates to a wiring board. In particular, the invention relates to a wiring board of excellent heat dissipating properties which is capable of being applied to power source equipment for industrial use.
Semiconductor modules employed in power source devices are applied over a wide range, from domestic equipment such as domestic air-conditioners or refrigerators, to industrial equipment such as inverters or servo controllers. In particular, a metal base wiring board or ceramic wiring board, which has excellent heat dissipating properties, is employed in for example IGBT (Insulated Gate Bipolar Transistor) modules incorporating power semiconductors, on account of the large amount of heat which these generate.
FIG. 3 is a view showing the cross-sectional construction of a conventional metal base wiring board.
A metal base wiring board has a three-layer structure comprising base metal 101, an insulating layer 102 formed on top of this base metal 101, and a circuit pattern 103 formed on top of this insulating layer 102. For the base metal 101, metal of excellent heat dissipating properties such as aluminum sheet or copper sheet, is employed. The insulating layer 102 comprises for example epoxy resin containing an inorganic filler such as SiO2, Al2O3 or AlN.
For the circuit pattern 103, usually, copper foil is employed, but sometimes aluminum foil is employed. As the copper foil, usually copper foil of thickness about 35 μm to 140 μm is employed. This copper foil is processed to form a prescribed circuit pattern using wet etching. In the case of a power semiconductor whose current capacity is small at about 10A and that generates little heat, the power semiconductor can be directly mounted by soldering to this circuit pattern 103. In the case of a power semiconductor of large current capacity, the thickness of the copper foil is increased to about 140 μm, in order to reduce the thermal resistance by spreading the heat out over this circuit pattern 103. If 140 μm is insufficient, even thicker copper foil such as for example copper foil of thickness 200 μm or 250 μm may be employed. Furthermore, if the thickness of the circuit pattern 103 is more than 1 mm, for example if a thickness of 3 to 4 mm is employed, a heat spreading effect is exhibited, allowing the heat generated in the power semiconductor to spread in the transverse direction and thereby greatly reducing the thermal resistance.
The insulating layer 102 that is used in a metal base wiring board needs to have excellent insulation reliability and heat dissipating properties. Furthermore, the insulating layer 102 needs to have excellent stress moderating properties, resistance to humidity and heat withstanding ability and the like; resin compositions which are suitable in this respect are known (see for example Laid-open Japanese Patent Publication No. 2002-12653, Laid-open Japanese Patent Publication No. 2002-76549 or Laid-open Japanese Patent Publication No. 2002-114836). In this way, a metal base wiring board is employed as a wiring board on which are mounted components that generate a lot of heat, such as power semiconductors, by bonding of the circuit pattern 103 with base metal 101 with interposition of an insulating layer 102 of excellent heat dissipating properties.
However, in the case of epoxy resin containing inorganic filler such as SiO2, Al2O3 or AlN, there are limits to the filler content, so the thermal conduction coefficient of such material is currently about 7 to 10 W/m·K. There are therefore also limits to the current capacity of the power semiconductor modules in which these can be employed; currently they can only be employed in modules up to about the 50 A class.
Accordingly, in the case of power semiconductor modules of larger capacity exceeding 50 A, ceramics wiring boards are employed, which have higher thermal conductivity of the insulating layer, rather than metal base wiring boards.
FIGS. 4a and 4b show the cross-sectional structure of a conventional ceramics wiring board, with FIG. 4a showing a ceramics wiring board and FIG. 4b showing a ceramics wiring board with base metal bonded thereto.
The ceramics wiring board is constructed by attaching a circuit pattern 103 onto both sides of a ceramics insulating plate 104. The ceramics insulating plate 104 is manufactured by kneading raw-material powder with a binder to produce an insulating plate in the form of a sheet, called a “green sheet,” and firing this at high temperature. After this, the wiring board is produced by bonding copper foil or aluminum foil for the circuit pattern 103 thereto at high temperature. Furthermore, this ceramics wiring board is usually bonded by means of a solder layer 105 to base metal 101 consisting of copper sheet of thickness about 2 to 3 mm.
For example Al2O3, AlN or Si3N4 are employed as the raw material for the ceramics insulating plate 104. The thermal conductivity of this ceramics insulating plate 104 is about 20 W/m-K in the case where the raw material is Al2O3, is about 60 to 180 W/mK in the case where the raw material is AlN, and is about 80 W/mK in the case where the raw material is Si3N4; i.e., its thermal conductivity is one to two orders of magnitude higher than in the case where inorganic filler is blended with epoxy resin.
However, in the case of metal base wiring boards, the thermal resistance is reduced, so, if thick copper foil is employed, the time required for the etching processing for processing the circuit pattern layer is increased in proportion to the thickness; processing costs are thereby greatly increased, giving rise to the problem that overall costs are greatly increased. Furthermore, if the thickness of the circuit pattern layer reaches 3 to 4 mm, a long time is required for melting of the copper and, in addition, etching of the edges of the circuit pattern layer cannot be achieved with high precision, so the etching process itself becomes impracticable.
A further problem was that, in the case of a ceramics wiring board, many steps were necessary, involving first manufacturing a ceramics insulating board, bonding this with a circuit pattern, etching processing, and bonding the ceramics wiring board manufactured in this way with the base metal by soldering, thus increasing the price and making it difficult to achieve price reduction. Furthermore, in the case of a ceramics wiring board, it is not possible to make the copper foil for the circuit pattern very thick. Although thick copper foil or copper sheet may be attached in order to increase the heat spreading effect, the copper sheet must be bonded with the ceramics insulating sheet at high temperature of about 1000° C. or more. If these two thicknesses are not made the same, warping is produced by the bimetal effect on cooling, due to the difference in thermal expansion coefficients. Also, as described above, if the copper foil or copper sheet is made thick, the cost of the etching process is greatly increased. So, in fact, no ceramics wiring boards having a circuit pattern thickness of more than about 0.6 mm are employed, and currently, a circuit pattern having a thickness less than 0.6 mm is used in the ceramic wiring board.
An object of the present invention is to provide a wiring board of low cost and excellent heat dissipating performance, which can be manufactured with few steps.
Further objects and advantages of the invention will be apparent from the following description of the invention and the associated drawings.